1. Field of the Invention
This invention relates in general to offshore drilling and construction operations and more particularly to an improved system for lowering tubular members and other equipment through a restricted opening at or below the sea bottom. On land, when a well is drilled for the production of oil and gas, the casing is lowered through the hole to seal the side of the hole, for example. This is not too complicated an operation because the drilling rig is adjacent the hole. However, in lowering operations involving a floating drilling vessel in water which may be several hundred or more feet in depth, the lowering operation becomes more complicated. The casing string (or other equipment) is usually lowered or run by attaching a running tool to the upper end of the casing string and having that tool attached to a string of small diameter pipe which can be called the handling string. The casing is then lowered into the wellbore by the lowering of the handling string. When the casing has reached its lower objective, the running tool is released and is retrieved at the surface by pulling the small diameter pipe.
2. Prior Art--Present System
At the present time, most running apparatus used in offshore operations is based on a left-hand threaded member attached to the lower end of a handling string fitted with a right-hand connection (normal tool joints) between sections of the handling string. As the casing string and/or other equipment reaches the desired position below sea level, the running tool can be released by rotation of the handling string to the right which disconnects the running tool without the risk of disconnecting the various joints of the handling string.
The increase in offshore drilling and construction activities from floating units resulting in displacement of the floating unit from directly above the wellbore axis and difficulties in placing the connecting point of the running tool in neutral axial loading due to the heaving of the vessel dictated the need for a more efficient subsea running tool. The above-mentioned vessel movements increase the torque required to disengage presently designed running tools from the subsea installation. In deep waters, the high torque required to disengage the running apparatus may result in damage of the connecting threads or twisting the handling string to failure. This is especially true when the tubular member to be run has a larger diameter than the handling string which is usually the case.
Various attempts have heretofore been made to overcome these limitations of existing equipment. In one prior art system, a running tool or apparatus contains a handling string swivel mounted to the tool body to accommodate displacement of the floating unit from above the wellbore axis in misalignment due to a sloping seafloor.
The foregoing type of running apparatus is considered to be less than satisfactory from a number of standpoints. First, the swivel permits only a limited degree of misalignment. Secondly, the torque transmitted through an inclined handling string will generate higher friction between the threaded elements. Third, the amount of the handling string inclination or misalignment becomes limited if the seat point is a few feet inside the wellbore or restricted opening.
In accordance with a second prior art system, a running tool with central torque actuating locking rams is used to reduce the torque required for disengaging the running tool from relatively larger diameter pipes. This type of running apparatus has not completely overcome the limitations set out above.
A prior art search revealed no art which showed the present invention. Four patents that were set forth in the report of that search had some but not all of the features of the present invention. These prior art patents are:
1. U.S. Pat. No. 3,222,088, J. A. Haeber, inventor, issued Dec. 7, 1965, "Wellhead Connector With Diagonally Directed Latches." This patent discloses the use of sliding blocks having grooved surfaces that mesh with grooves on the outer surface of the inner member to which it is to be attached. The grooves are described as being continuous or interrupted circumferential grooves. Attempting to turn one of these members with respect to the other could not unthread the tool. PA0 2. U.S. Pat. No. 3,321,217, A. G. Ahlstone, inventor, issued May 23, 1967, "Coupling Apparatus for Wellheads, and the Like." This shows a latching device similar to the one in the Haeber patent supra. PA0 3. U.S. Pat. No. 3,456,729, J. M. Harwell, Jr., inventor, issued July 22, 1969, "Stab-In Conduit Couplings." This describes a conduit coupling that consists of an outer slotted member that has internal left-hand threads cut in it which match with similar threads on the outside surface of the male member. Engagement is made by forcing the inner member downward and springing the slotted portion outward to permit the threads to pass each other until full engagement is acquired. No latching dogs are involved. PA0 4. U.S. Pat. No. 3,675,713, Bruce J. Watkins, inventor, issued July 11, 1972, "Method and Apparatus for Separating Subsea Well Conduit Coupling From a Remote Floating Vessel." This describes a connector very similar to the one Haeber reference described above and uses a series of hydraulic cylinders mounted in a manner to force the coupling apart in the event it jams due to misalignment and cannot be separated after the locking dogs are withdrawn. There is no provision for separating by unscrewing in event the locking dogs cannot be disengaged.